A circuit board mechanically supports and electrically connects electronic components using conductive traces, pads and other features etched from electrically conductive sheets, such as copper sheets, laminated onto a non-conductive substrate. Multi-layered circuit boards are formed by stacking and laminating multiple such etched conductive sheets and non-conductive substrates. In some applications, high performance rigid and flexible circuit board-based interconnects are constructed using geometries that form one or more transmission lines within the circuit board. The transmission lines are formed from a patterned conductive layer positioned within the circuit board stack-up. A geometry consisting of a single transmission line is referred to as a single ended geometry. A geometry consisting of two transmission lines is referred to as a differential pair geometry. In order for the transmission lines to meet specific impedance characteristics, the circuit board stack up also includes one or more return planes, one return plane stacked above and/or another return plane stacked below the transmission lines. A transmission line geometry refers to a specific combination of the one or more transmission lines and the one or more return planes. Two types of transmission line geometries that are commonly used in rigid and flexible circuit boards are the stripline and microstrip geometries. The stripline geometry refers to the transmission line geometry having two return planes. The stripline geometry can be combined with either a single transmission line, collectively referred to as a single ended stripline geometry, or two transmission lines, collectively referred to as a differential pair stripline geometry. The microstrip geometry refers to a transmission line geometry having a single return plane. The microstrip geometry can be combined with either a single transmission line, collectively referred to as a single ended microstrip geometry, or two transmission lines, collectively referred to as a differential pair microstrip geometry. In either the microstrip or stripline geometries, the return planes are constructed out of continuous, solid planes of conductive material.
FIG. 1 illustrates a conventional circuit board 14 having a differential pair stripline geometry. The left hand side shows a cut out side view of the circuit board 14 as a laminated stack. The right hand side shows a perspective exploded view of the circuit board 14. The circuit board 14 includes an upper return plane 2, one or more isolation layers 10, two signal traces 6 and 8, one or more isolation layers 12, and a lower return plane 4. The two signal traces 6 and 8 form the differential pair transmission lines. The one or more isolation layers 10 and the one or more isolation layers 12 can be made of different non-conducting dielectric materials and thicknesses, which prevent the signal traces 6 and 8 from electrically shorting to the return planes 2 and 4. The upper return plane 2 and the lower return plane 4 each form continuous, solid planes. The return planes can be made of one or more conductive materials, including, but not limited to, copper, nickel, gold, and tin, or a mixture of conductive and non-conductive materials, such as conductive ink.
FIG. 2 illustrates a conventional circuit board 16 having a single ended stripline geometry. The left hand side shows a cut out side view of the circuit board 16 as a laminated stack. The right hand side shows a perspective exploded view of the circuit board 16. The circuit board 16 having the single ended stripline geometry is structurally similar to the circuit board 14 having the differential pair stripline geometry of FIG. 1 except the circuit board 16 has only a single trace 6 instead of two traces.
FIG. 3 illustrates a conventional circuit board 18 having a differential pair microstrip geometry. The left hand side shows a cut out side view of the circuit board 18 as a laminated stack. The right hand side shows a perspective exploded view of the circuit board 18. The circuit board 18 having the differential pair microstrip geometry is structurally similar to the circuit board 14 having the differential pair stripline geometry of FIG. 1 except the circuit board 18 has only a single return plane 4 instead of two return planes. The one or more isolation layers 10 are optional in the case of the circuit board 18, whereas the one or more isolation layers 10 are mandatory in the case of the circuit board 14 of FIG. 1.
FIG. 4 illustrates a conventional circuit board 20 having a single ended microstrip geometry. The left hand side shows a cut out side view of the circuit board 20 as a laminated stack. The right hand side shows a perspective exploded view of the circuit board 20. The circuit board 20 having the single ended microstrip geometry is structurally similar to the circuit board 16 having the single ended stripline geometry of FIG. 2 except the circuit board 20 has only a single return plane 4 instead of two return planes. The one or more isolation layers 10 are optional in the case of the circuit board 20, whereas the one or more isolation layers 10 are mandatory in the case of the circuit board 16 of FIG. 2.
In some applications, a circuit board having transmission line(s) requires a high degree of flexibility. However, return plane(s) that are continuous, solid planes are too rigid to meet many such flexibility requirements. In such applications, each continuous, solid return plane can be replaced with a cross-hatched return plane. Cross-hatching involves removing a portion of the conductive material in the return plane so that the return plane is no longer a continuously solid plane. A circuit board having cross-hatched return plane(s) has a much greater degree of flexing, twisting, bending, and the like than a circuit board having continuous, solid return plane(s). The cross-hatched return plane has a cross-hatch width W equal to a cross-hatch height H, as shown in FIG. 5, and the cross-hatch pattern is randomly placed relative to the transmission lines. Although the cross-hatched return plane(s) enables a circuit board to have a greater degree of flexibility, cross-hatching degrades the electrical performance of the microstrip and stripline transmission line(s).